I've got a copy of Alfred Watkins' 1925 book "The Old Straight Track" which I picked up at the Megalithomania conference but haven't read it yet (still stuck in the middle of All Done with Mirrors).

Watkins never visited the Stonehenge area, but flicking through the index of his book I see he advanced the idea that Sidbury was involved in a Stonehenge alignment, but in the appendix he reports he's been informed that Stonehenge and Sidbury aren't intervisible.

Idle musing - I wonder if you could see Sidbury if you stood on the top of the lintel ring at Stonehenge, or on the even higher great trilithon (of which only one stone still stands). It'd put you at least 30 feet higher off the ground.
--
simon